American Scientist

Twilight of the Mammoths: Ice Age Extinctions and the Rewilding of America. Paul S. Martin. xx + 250 pp. University of California Press, 2005. $29.95.

Between 50,000 and 10,000 years ago, during the final millennia of the Pleistocene Epoch, roughly 100 genera of megafauna (animals weighing more than 100 pounds) became extinct worldwide. Among them are such well-known creatures as mammoths and saber-toothed tigers and the more obscure, though no less significant, Diprotodon (an Australian marsupial the size of a hippopotamus) and Coelodonta (a woolly rhinoceros found in Europe). Whether their disappearance was caused by changes in climate or by "overkill" (being hunted to extinction by humans) has been hotly debated for the past 40 years. In Twilight of the Mammoths: Ice Age Extinctions and the Rewilding of America, Paul S. Martin reviews the end-Pleistocene extinction, arguing that overkill is the more likely explanation.

In the first chapter Martin introduces the reader to the Pleistocene, evaluating the taxa that were present on each continent and summarizing what we know about those that became extinct. He has helpfully included figures containing scaled drawings of reconstructed skeletons and sketches of some of the extinct megafauna. The second chapter summarizes the overkill hypothesis and notes that accurate dating is needed to further develop the concept.

In the next few chapters Martin provides an autobiographical account of his fieldwork and of the evidence for overkill he has gathered during his long career. He recounts his experiences working with colleagues and friends in various caves in North and South America. We learn about the vast deposits of ground-sloth dung found both in Rampart Cave in Arizona and in Cueva del Milodón in Chile. Martin also describes the evidence from cave deposits that ancient condors and mountain goats were typical inhabitants of the Grand Canyon. He highlights the synchronous extinction of large mammals in North and South America around 12,000 years ago and discusses the ecology of many of the extinct mammals—explaining, for example, how pollen and microfloral analyses of dung from Rampart Cave reveal that the globe mallow plant was the preferred food of the extinct Shasta ground sloth.

Succeeding chapters lay out the dynamics of the extinctions, including the effects of human dispersal into new areas, the chronological relation between the arrival of humans and the disappearance of large mammals, and the evidence at kill sites that megafauna were used by humans. Martin makes a strong case that anthropogenic influences caused the demise of megafauna in North America, citing evidence that the continent's first human inhabitants, the Clovis people, arrived about 13,000 years ago, just before the extinction episode. Although the North American data may not be entirely convincing on its own, when Martin adds to it a discussion of the arrival of humans and subsequent extinction of megafauna in Australia, Madagascar, New Zealand, South America and the West Indies, his argument in favor of the overkill hypothesis becomes persuasive.

A separate chapter deals with the possibility that the extinctions were induced by climate change. Martin dismisses the idea that climate could on its own provide a viable explanation, arguing that shifts in temperature at the end of the Pleistocene were not much different from those that had occurred earlier in the epoch, when few if any megafauna became extinct. Martin explains his position sufficiently. However, scientists are aware that climate change does affect mammals (by changing population size, for example), and the chapter would have benefited from a more detailed discussion of this. Recent studies have analyzed ancient DNA  from radiocarbon-dated fossils of both large and small mammals, determining how genetic diversity has changed over time. In some cases, a great decrease in genetic diversity, suggesting a severe reduction in population size, has been shown to have taken place at about the same time as a shift in climate. A sharp reduction in the population size of a species could in turn play a role in its ultimate disappearance. Investigating these types of effects at the end of the Pleistocene would greatly enhance our understanding of exactly how the extinction proceeded.

Conservation is a theme throughout the book, and Martin closes with an overt call for drastic conservation efforts on behalf of extant fauna. He also shows the importance of fossils—whether bones, sediment or dung—as a nonrenewable resource. They contain valuable information that can easily be lost, as when sloth dung in Rampart Cave was inadvertently burned.

However, Martin's suggestion that we should create Pleistocene parks in North America seems rash and extreme. Introducing nonnative animals such as camels, elephants and lions to establish a prehuman ecological state could have dire consequences. Many of these mammals—elephants, for example—diverged evolutionarily from their extinct North American counterparts millions of years ago, claiming their own niches and coevolving with other non-North American species. Although we speculate that at times the living animals behave like extinct forms, introducing these species will not recreate the "wild" state that was lost when the megafauna died. Interactions between the native and introduced species would be unfamiliar and would have to evolve anew. A better solution would be to focus on preserving remaining fauna and environments.

Martin's ill-conceived ideas about Pleistocene parks notwithstanding, Twilight of the Mammoths is well written and can be easily understood by nonscientists. It provides a fine introduction to the end-Pleistocene extinction through the eyes of a proponent of the overkill hypothesis. I particularly enjoyed the personal accounts of fieldwork and of interactions with colleagues. Also, the subsections that close some of the chapters are very insightful. In fact, the material on radiocarbon dating is so succinct and informative that I plan on using it for teaching. Natural history enthusiasts will certainly enjoy this book.